In recent years, a technology capable of improving a transmission speed without increasing a transmission band and a transmission power by employing a plurality of antennas in a transmitter/receiver, which is called MIMO (Multiple-Input Multiple-Output), attracts attention in a radio communication system.
The MIMO is a speed-up technology capable of improving a communication capacity by multiplexing and transmitting different pieces of data from a plurality of transmission antennas, separating this multiplexed signal in a plurality of reception antennas having received the above signal, and extracting the different pieces of data.
Such a signal separation is carried out by utilizing an inverse matrix of a channel matrix having a propagation gain in an antenna pair, which is decided by a combination of each transmission antenna and each reception antenna, as a component. For this, it is desirable from a viewpoint of the signal separation that the propagation gain differs for each antenna pair (that is, it is desirable that a correlation characteristic of the channel matrix is small).
For example, when many paths arrive between the transmission antenna and the reception antenna from various directions, a correlation characteristic of the channel matrix becomes small because a phase relation between the path partners differs for each antenna pair. Thus, a separation characteristic of the signal is improved, and an effect of enlargement of a communication capacity (enlargement of a system capacity) is acquired. From now on, this effect will be described as “an effect of the MIMO” in some cases.
On the other hand, when only one path arrives between the transmission antenna and the reception antenna, a value of the propagation gain becomes identical for each antenna pair, and it becomes difficult to separate the signal. As a result, a sufficient effect of the MIMO becomes difficult to acquire.
As described above, the effect of the MIMO depends upon a radio wave arrival status (a radio wave propagation characteristic) between the transmission antenna and the reception antenna. For this, it is necessary to pre-evaluates the degree of the effect of the MIMO in an introduction area, and appropriately decide an installation position of a base station antenna at the moment of introducing the radio communication system adopting the MIMO. The system for estimating the effect of the MIMO over a computer having an environment of the introduction area simulated therein is employed in order to makes such an antenna installation design.
Conventionally, the following technique exists as a technique of estimating the effect of the MIMO over this computer.
For example, the technology of deriving the effect of the MIMO by employing a ray tracing method is known (Non-patent document 1). Herein, the so-called ray tracing method is a technique in which a radio wave being radiated from an antenna is represented by a number of radio wave lines (rays), and rays that arrive at the reception point are synthesized to obtain a propagation loss and a delay amount on the assumption that each ray is propagated while repeating reflection and transmission geometrically-optically.
Additionally, while the ray tracing method can be classified broadly into a ray launching method and an imaging method, both of these are applicable so far as the technique disclosed in the Non-patent document 1 is concerned.
Herein, the so-called ray launching method is a technique of searching a locus of the ray one by one on the assumption that the ray radiated discretely from the transmission antenna at a constant angle is propagated while repeating the reflection and the transmission in a construction and an object.
Further, the imaging method is a technique for determining a reflection path of the ray, which connects the transmission and reception points, while obtaining a mirror image point for a reflection plane of a transmission path of the ray, which connects the transmission and reception points. The imaging method can realize a higher estimation precision as compared with the ray launching method because it can search the vigorous propagation path of the ray between the transmission point and the reception point. The details of the ray launching method and the imaging method are disclosed, for example, in Non-patent document 2 and Patent document 1 as well.
By the way, in the technique disclosed in the Non-patent document 1, at first, the situation of topography and buildings in the introduction area is simulated, and a propagation path ranging from the transmission antenna to the reception antenna is obtained for each antenna pair by employing the ray tracing method. Next, the channel matrix is obtained from the acquired propagation path, and a propagation loss in a logical path of the MIMO is calculated from an eigenvalue thereof. In addition, SNR (signal-to-Noise Ratio) is calculated for each logical path from the acquired propagation loss, and a throughput at the time of applying the MIMO is calculated based thereupon. And, by comparing the acquired throughput with a throughput in the case of not applying the MIMO, the effect of the MIMO is derived
However, with the method of the Non-patent document 1, a problematic point is disclosed in which an arithmetic processing amount at the moment of the calculation becomes enormous, and hence, a processing time is increased. Particularly, in the case of planerly estimating the effect of the MIMO in the evaluation area in the adjacent of the base station antenna, that is, in the case of defining the transmission point and a plurality of the reception points in the adjacent of the transmission point, and estimating the radio wave propagation characteristic between them, or the like, there exists a problem that a time required for the estimation (analysis) becomes enormous.
For example, in the case of obtaining the propagation path ranging from the transmission antenna to the reception antenna by employing the ray launching method, obtaining an accurate propagation path necessitates the estimation that takes the effects such as diffraction and irregular reflection into consideration. However, in the case of performing the ray launching method while taking these effects into consideration, an analysis time is enormously increased as compared with the case that these effects are not taken into consideration. Further, in the case of obtaining the propagation path ranging from the transmission antenna to the reception antenna by employing the imaging method, much analysis time is originally required also when the number of the pairs of the transmission point and the reception time is only one, and besides it, it takes a long time for the analysis all the more in the case of performing the estimation in a planerly area because the analysis with the imaging method have to be performed for a large number of the reception points as well.
Thereupon, the technique of reducing an arithmetic processing amount has been proposed so as to solve such a problematic point (Patent document 1). In the technique of the Patent document 1, the arithmetic processing amount accompanied by the searching of the propagation path of the ray is reduced by reducing the structures that are taken into consideration at the moment of estimating the propagation, or by simplifying a shape of the structure. Specifically, the technique of the Patent document 1 selects and stores buildings in advance that exist in a road and an intersection, in a street microcell in which base stations are arranged on a road and service areas are formed along the road, and performs the propagation estimation by taking only the above buildings into consideration.
However, this technique of the Patent document 1 causes a problem that the estimation precision has to be sacrificed in exchange for a reduction in the arithmetic processing amount because the structures that are taken into consideration are reduced, or a shape of the structure is simplified at the moment of estimating the propagation.
The technique of performing the high-precision propagation estimation while taking the diffraction into consideration without remarkably increasing the arithmetic processing amount has been proposed as a solution to such a problem (Patent document 2). The Patent document 2 discloses the radio wave propagation characteristic estimating system for estimating the propagation characteristic of the radio wave that goes from the transmission point decided within a limited evaluation area up to a plurality of the reception points within the foregoing evaluation area. This radio wave propagation characteristic estimating system includes a non-diffracted wave estimating means for estimating a component other than a diffracted wave out of the components of the radio wave that goes from the transmission point up to the respective reception points, a diffracted wave estimating means for estimating a component of the diffracted wave out of the components of the radio wave that goes from the transmission point up to the respective reception points, and a total radio wave component calculating means for calculating a total radio wave propagation characteristic in the foregoing each reception point while taking an estimated result in the foregoing non-diffracted wave estimating means and an estimated result in the foregoing diffracted wave estimating means into consideration.
The technology of the Patent document 2 described above realizes the high-precision propagation estimation while taking the diffraction into consideration without remarkably increasing the arithmetic processing amount by applying the high-speed and yet high-precision radio wave propagation estimation method for the radio wave propagation estimation of the non-diffracted wave, and employing the radio wave propagation estimation technique of which the arithmetic load is light for the diffracted wave.
Non-patent document 1: K. H. Ng et al. “Efficient Multielement Ray tracing with Site-Specific Comparisons Using Measured MIMO Channel Data” IEEE Trans. Vehicular Technology, Vol. 56, No. 3, pp. 1019-1032, 2007
Non-patent document 2: Yosio Hosoya (editorial supervision) “Radio Wave Propagation Handbook”, REALIZE INC., pp. 234-243, 1999
Patent document 1: JP-P1997-33584A
Patent document 2: JP-P2007-101376A